Process for the beneficiation of phosphate rock



PROCESS FOR THE BENEFICIATION OF PHOSPHATE ROCK Filed Oct. 11, 1963 MATRIX INVENTORS J.D. CLARY E- O'BRIEN NOTARY i/MEM ATTORNEY United States Patent 3,326,474 PROCESS FOR THE BENEFICIATION 0F PHOSPHATE ROCK Joe D. Clary, Laireland', Fla, and Ellis J. OBrien and Joseph A. Notary, Pittsburgh, Pa, assignors to W. R. Grace & (10., New York, N.Y., a corporation of Connecticut Filed Get. 11, 1963, Ser. No. 315,529 7 Claims. (Cl. 241-) This application is directed to the beneficiation of phosphate ores. In particular, this invention is directed to a novel process for separating the valuable phosphate rock from the clay and sand contained in naturally occuring phosphate ores, wherein the phosphate ore is processed while in a substantially dry condition.

When phosphate ore is mined from the earth, it is referred to as a matrix. This matrix is comprised of pieces of phosphate rock and silica which are admixed in a claylike material which is denoted as slimes. In order to obtain a phosphate rock which is usuable in the production of fertilizer products, such as superphosphate, etc., or other products such as phosphorous or phosphoric acid, it is necessary to remove substantially all of the siliceous and clay-like material from the matrix.

The prior art has disclosed numerous processes and means which have been developed to obtain a phosphate rock which is reasonably free of slimes and silica. In general, most 011 of these processes utilize a complex system of screening and surface washing in conjunction with table and froth flotation steps which further increase the efliciency of the recovery process.

In order to operate an efiicient recovery process, the matrix material must be disintegrated as such as possible prior to attempting to recover the phosphate rock from the silica and clay. In order to distintegrate the matrix, the present commercial practice is to slurry the matrix with water and then to subject it to a series of screening, abrasion, and washing steps. The matrix will then be disintegrated sufliciently to permit the subsequent removal of the sand and slimes through the use of conventional phosphate recovery processes and apparatus. Such processes usually involve first screening the distintegrated matrix to recover the large phosphate rock particles and to separate the extremely fine (-150 mesh) particles and slimes from the matrix. The slimes and fine particles are disposable materials. The fine material (1 mm. to +150 mesh) which passes through the initial screening step is then subjected to table or froth flotation processes to recover small and intermediate sized phosphate particles. In the table flotation process, the 20 +48 mesh fraction of the matrix is subjected to a complex treatment with anionic flotation reagents, which are usually a mixture of fatty acids, kerosene, fuel oil, and caustic soda. As used commercially, the froth flotation process is a double flotation operation. In the first froth flotation step, the 35 +150 mesh fraction of the matrix is treated with a reagent comprising caustic soda, fuel oil, fatty acid, and, in some operations, a minor amount of kerosene. The concentrate from this flotation step is then cleaned Wit-h sulfuric acid to remove oil and traces of fatty acid, washed, and conveyed to the next flotation step in which amines are added as reagents. From this second flotation step, a phosphate concentrate is obtained which possesses very high bone phosphate of lime (BPL) values.

One of the major problems which is encountered in using the above-described process is that of the disposable clay slimes and fine siliceous particles which are separated from the phosphate matrix. These slimes must be separated from the matrix prior to subjecting the matrix to the table and cell flotation treatments, for the slimes have 3,325,474 Patented June 20, 1967 extremely large surface areas and tend to absorb chemical reagents, thereby thereby making the flotation cost prohibitive. The slimes also contain considerable amounts of phosphate, generally analyzing in the range of from 20-25% BPL. It is therefore highly desirable to provide a process for the benefication of phosphate ores in which this objectionable disposal product could be eliminated.

Another acute problem which is encountered in using the beneficiation process mentioned above is that of the siliceous and clay-like material adhering to the surface of the phosphate rock. The operating efiiciency of this process is dependent upon the removal of Substantially all of these materials from the phosphate rock. The siliceous and clay-like materials which are present in the original phosphate matrix may also take the form of mud balls when the matrix is slurried with water and pumped to the washing plant and carried through to the subsequent processing steps with the pieces of phosphate rock of substantially the same size. The mud balls are of two types; an admixture of clay and silica alone, and an admixture of clay, silica, and phosphate particles. The mud balls cause extensive clogging of the screening devices and will result in a loss of a considerable amount of phosphate rock. The mud balls are of two types; a mixture of clay and silica or a mixture of clay, silica, and phosphate rock particles. The mud balls will produce excessive clogging during screening and will cause the loss of a considerable amount of valuable phosphate material. To eliminate the problem of mud balls and surface adhesion, and to clean the surface of the phosphate rock, surface washings and expensive disintegration and separation apparatus must be employed.

It is an object of this invention therefore to provide a novel process for beneficiating phosphate ores whereby the above-mentioned problems of the prior art are avoided. Specifically, it is an object of this invention to provide a substantially dry process for efficiently and thoroughly disintegrating a phosphate matrix into its major component parts, thereby effectively preparing the matrix for use in a subsequent phosphate recovery process. Other objects, advantages, and features of this invention will be apparent to those skilled in the art in view of the following more detailed description of the invention.

These and other objects are achieved by means of this invention by which is provided a substantially dry process for the beneficiation of phosphate rock. In particular, the process comprises thermally drying a phosphate matrix while simultaneously subjecting the matrix to an attrition effect, air-classifying the dried matrix to produce pebble phosphate and fine particles of phosphate rock, silica, and agglomerated clay, and separating the pebble phosphate from the fine particles and recovering same as a final product. The fine particles of phosphate rock are then electrostatically separated from the time particles of silica and agglomerated clay and subsequently recovered. By proceeding in the above manner, the problems of the disposable slimes and the objectionable mud balls will be completely eliminated.

The invention will be further understood by referring to the accompanying drawing. It should be understood that this drawing is intended to be only a means of illustrating the inventive concept and is not to be considered a limitation of same.

In the drawing, the numeral 1 designates a feed hopper and means to which the phosphate matrix is conveyed through line 2 fromthe mine and from which said matrix is conveyed to the attrition column 3. Positioned beneath the column 3 and in direct communication therewith is a fluidized bed chamber 16. An impingement chamber 4 is positioned above the attrition column and connects said column with a first scrubber collector means 5.

The design of the impingement chamber is important. The angle between the impinged surface and the direction of flow of material (gas and solid) impinging on said surface must be sufliciently sharp to produce therequired disintegration of the matrix. In general, said angle may be within the range of from 60 to 150, with the preferred angle being 90. In lieu of the impingement chamber, however, the disintegration chamber may be any autogenous grinding means, such as a Jordan mill or a ball milling device, can be used, if desired. Any such means is suitable so long as it is capable of imparting sufficient disintegration to the matrix. It is to be understood, therefore, that the scope of this invention is intended to include the usage of all such devices.

Pneumatic classification occurs within first scrubber 5, whereby the matrix is separated into a fine fraction with a maximum particle size of about 150 to about 325 mesh and a corresponding coarse fraction ranging from about +150 to about +325 mesh. (As used throughout this application, mesh size refers to the U. S. Standard.) The oversize fraction is taken to a dry screen separation means 6 wherein the pebble phosphate material (+14 mesh) is removed through line 7 as a final product. The -14 mesh material, comprising fine particles of phosphate rock, silica, and a small amount of agglomerated clay, is removed from the screen separator 6 through line 8 and conveyed to an electrostatic separator 9. Within the electrostatic separator 9, the matrix is divided into a phosphate rock product which is removed through line 10 and into a disposal product comprising the silica and the agglomerated clay which is removed from the separator through line 11. The electrostatic separator can also produce a middling product, i.e., phosphate rock containing suflicient siliceous and clay material to make the phosphate rock an unsuitable product, but with the phosphate rock particle being sufliciently large to preclude its separation into the disposable material. The middlings will be removed from the separator through line 12 and recirculated to the electrosatic separator 9 in which they will be reprocessed as described. Alternatively, depending upon the quantity of silica and clay in the middlings, they may be recirculated to feed means 1, via broken line 22 and manipulation of valve 23, for further processing.

The electrostatic separation device employed in this invention can be any of those which are presently commercially available. Examples of suitable separators are those described in US. Patents 2,357,658 and 2,738,067 or any of the devices listed in Taggart, Handbook of Mineral Dressing, pages 40-47.

The aforesaid fine fraction, which is comprised primarily of particles of agglomerated clay, is removed from the scrubber collector 5 and conveyed through line 20 to a second scrubber collector 13 wherein approximately 95% of the fine material will be recovered and collected. This material will subsequently be removed from the collector through line 15. The gases from this collector will be wet-scrubbed and exhausted to the atmosphere through line 14.

Line 17 is connected to the fluidized bed chamber 16 for removing therefrom any phosphate matrix which is not sufiiciently disintegrated within the attrition column and the fluidized bed chamber. This material will be taken to a dry screen 18. The inch material will be removed from the screen through line 21 as a final product. The inch material will be removed from the screen through line 19 and recirculated to the feed means.

The process of this invention operates generally as follows. Phosphate matrix is fed through line 2 to the hopper and feed means 1 from which it is fed to the attrition column 3. Hot gases at a temperature of approximately 350 F. and at a velocity of approximately 7,000 feet per minute flow from the fluidized bed chamber up through the attrition column 3. The velocity of the gases is adjusted such that the matrix which is inch will be forced up the column and into the impingement chamber 4 whereas the inch matrix will fall down the attrition column and into the fluidized bed chamber 16, thereby effectuating pneumatic classification of said matrix (into inch and inch particles) within said attrition'column. (As used in this application the term pneumatic classification means the separation of solid material into cuts, or fractions, of desired particle size, or mesh, via a stream of inert gas or gaseous fluid. Stream flow is used to produce said classification within attrition column 3, and centrifugal flow is used for said purpose within first scrubber collector 5. Said inert gas can be any gas or gaseous mixture that does not react with said matrix. Suitable gases include, but are not limited to, air, nitrogen, helium, argon, and combustion gases comprised of nitrogen, carbon dioxide, water vapor, argon.) The inch matrix is forced through the impingement chamber into first scrubber collector 5. Pneumatic classification also occurs within said first collector, whereby the matrix is separated into a fine fraction ranging from about to about 325 mesh and a corresponding coarse fraction ranging from about +150 mesh to about +325 mesh. Said coarse fraction is removed from scrubber collector 5 to dry screen 6 which separates this portion of the matrix into +14 mesh final product pebble phosphate and a fraction which passes a 14 mesh screen comprising fine particles of phosphate rock, silica, and a small amount of agglomerated clay which is taken through line *8 to the electrostatic separator 9. The fine particles are electrostatically separated into a phosphate rock product, which is removed through line 10, and into a disposal product comprising the silica and agglomerated clay which is removed through line 11. As explained above, the separator may also produce a middling product, which is removed from the separator through'line 12 and recirculated for further processing.

It should be understood'that the electrostatic separation 7 may be a multiple pass operation, i.e., the phosphate concentrate and the disposal product may be recycled to the separator and reprocessed to produce a sufliciently high BPL content (7080%) in the concentrate and a sufficiently low BPL (0-5 in the disposal product. If required, these products may be recycled and electrostatically separated four or five times. Such an operation is intended to be within the scope of this invention.

A fine (ca. 150-325 mesh) fraction which is obtained from the scrubber collector 5 is removed through line 20 and conveyed to a second scrubber collector 13, wherein the fine clay-like material is recovered and collected. The gases from the collector are wet-scrubbed and exhausted to the atmosphere through line 14.

The inch phosphate matrix which falls down the attrition column and into the fluidized bed chamber Will be disintegrated during the fall by the action of the high temperature gases flowing upward from the bed chamber and into the attrition column. The material on the bed plate will be fluidized by the gases and dried. The clay material will be degraded by the agitation produced While in the fluidized state and will be lifted upward into a contraction chamber opening into the attrition column. This degraded material travels up the attrition column and is processed with the inch material as described above. The material on the bed plate which is not degraded is discharged from the bed plate through line 17 and passes through dry screen 1 8 wherein the material is separated into a inch fraction, which is removed through line 21 as a final product, and into a inch fraction which is removed through line 19 and recirculated to the feed means 1.

It is thus seen that by proceeding in the above-described manner, a completely integrated process and combination of apparatus is provided for the disintegration and benefication of phosphate ore. The success of this operation depends to a great extent upon the velocity and temperature of the fluid which passes through the fluidized bed chamber and attrition column. The temperature and velocity of this fluid must be adjusted so as to convey the inch material through the attrition column, impingement chamber, and scrubber collector, thereby disintegrating the material and preparing it for electrostatic separation. In general, the velocity of this fluid can be in the range of from about 4000 feet per minute to about 9000 feet per minute, with the preferred velocity being approximately 7,000 feet per minute. The temperature of the gaseous fluid within the attrition column can range from 250-400 F., with the preferred temperature being 350 F. To maintain this temperature within the column, a temperature range of about 20004000 F. should be maintained below the fluidized bed. The higher temperature is required below the fluidized bed in order to compensate for loss of heat due to the moisture contained in the matrix which falls down the column and into the fluidized bed chamber. Any substantially inert gaseous fluid is suitable for usage in this operation. Illustrative examples of such are air, combustion gases comprising water vapor, carbon dioxide, nitrogen, and oxygen, carbon dioxide, nitrogen, argon, helium, etc.

There are numerous advantages to the beneficiation process and combination of apparatus disclosed in this invention. Perhaps the greatest benefit to be derived is the elimination of the slime disposal problem. The present commercial practice is to pump the slimes into large ponds covering thousands of acres wherein the clay-like material settles to the bottom and the water is recycled for further usage. Since the slimes have a great tendency and capacity to absorb water (up to approximately 75-80% of their weight) the land on which this material is deposited becomes completely useless and worthless. Before the slimes can be disposed of, large dams must be built to retain the material. By utilizing this invention the expense of preparing the land for disposal and the waste of the land are eliminated entirely.

The process and apparatus of this invention eliminates the necessity of pumping the phosphate matrix as a wet slurry from the mine to the processing plant as is presently done. This invention makes it possible to transport the matrix by means of a truck or conveyor system directly from the mine to the feed hopper. This greatly simplifies the transportation requirements since the mine site may be a distance of from three to five miles or more from the processing plant. To pump the matrix over this distanoe and to dispose of the slimes and recycle the water requires a pumping system comprising over thirty miles of 16" piping (the cost of this is from $8 to $10/ft.), twelve $80,000 pumping units, and one-half million dollars of electricity per year. This invention, therefore, results in a very substantial reduction in operating costs.

This invention also eliminates the cell and table flotation steps which are complex and expensive treatments because of the reagents and equipment which must be used. This further reduces considerably the cost of the operation. The electrostatic separation produces a dry phosphate product which is ready for any of the commercial uses without the necessity of first storing and drying the product, thereby additionally reducing the capital investment, manpower and maintenance expenses.

In addition to the above advantages this invention elimi nates the necessary complex washing, screen and disintegration procedures which are used in the present commercial wet process to clean the surface of the phosphate rock and to disintegrate the mud balls. By eliminating these procedures, the operating and maintenance expenses are further reduced.

This invention will be better understood by reference to the following specific but non-limiting examples.

Example 1 Proceeding as described, Florida phosphate matrix was fed at a rate of approximately 1000 lbs. per hour into the attrition column. Hot (ca. 3000 F.) combustion gases comprising water vapor, carbon dioxide, nitrogen, and oxygen were introduced into the fluidized bed chamber and circulated at a velocity of approximately 7000 feet per minute through the attrition column, thereby producing an equilibrium temperature of about 350 F. in said column. Said combustion gases dried and conveyed all of the phosphate matrix of inch and smaller upward through the attrition column, into and through the impingement chamber, and to the scrubber collector, wherein the matrix was collected and separated into a --325 mesh fraction and a +325 mesh fraction.

The 325 mesh fraction was continuously removed from the scrubber collector and taken to a second collector. The gases from the collector were wet-scrubbed and exhausted to the atmosphere. The extremely fine material was removed from the collector as a dry by-product. Approximately 928 lbs. of fine material was recovered within the collector during the period of operation.

The +325 mesh fraction from the first collector was taken to a dry screen wherein it was separated into +14 mesh and l4 mesh fractions. Approximately 720 lbs. of the +14 mesh was taken off as the pebble phosphate final product. The l4 +325 mesh fraction, consisting essentially of fine particles of phosphate rock, silica, and a small amount of agglomerated clay, was taken from the screen in a substantially dry heated condition to the electrostatic separator.

Within the separator, the dried particles passed over a high tension rotor electrode (revolving at a speed of about 0.1- rpm. and having a potential of about 20,000-50,000 volts) which separated the feed into three distinct products, i.e., a phosphate rock concentrate, a disposal product comprising silica and a small amount of agglomerated clay, and a middling product which is recirculated and reprocessed. Operating over a period of 8 hours, 3,232 lbs. of phosphate rock and 2,720 lbs. of disposal product were produced by the separator.

Results of screen and chemical analyses of the feedstock are given in Table I, and similar data for the product (concentrate) are presented in Table II.

During the operation of this example, the inch phosphate matrix descended through the attrition column and into the fluidized bed chamber. A large proportion of material was disintegrated during the fall by the action of the hot, dry gases. This disintegrated material was forced up the attrition column and processed and collected as described above. Any material collected on the bed plate of the fluidized bed chamber which was not disintegrated was discharged from the fluidized bed chamber and passed through a dry screen wherein the material was separated into a inch fraction, which was removed as a final product, and into a inch fraction, which was removed from the screen and recirculated and reprocessed. During the period of operation, lbs. of inch product was recovered.

Example 2 The general procedure of Example 1 was repeated using the apparatus that was used in said example. However, in this instance, the equilibrium temperature in the attrition column was about 400 F. Results were substantially the same as those obtained in Example 1.

An analysis of the results of these runs proves that the process and combination of apparatus of this invention produced a phosphate rock product of sufliciently high BPL content to be suitable for usage in the production of elemental phosphorus, phosphoric acid, fertilizers, or any of the other commercial phosphate products.

It is to be understood that many equivalent modifications will be apparent to those skilled in the art from a reading of the foregoing disclosure without a departure from the intended concept of the invention.

TABLE I.FEEDSTOCK Weight Per- Assay cent of BPL Dis- Material Feedstock tribntion 4 Retained BPL 1 Insol- 1 dz A 3 in Fraction uble 2 nch Fraction 2.0 72.1 7. 5 2. 3 3. 5 Inch +14 Mesh Fraetion 9. O 74. 2 6. 4 2. 1 16. 2 -14X+325 Mesh Fraction 77. 4 38. 56. 9 2. l 71. 3 -325 Mesh Fraction l1. 6 32. 1 43. 2 13. 6 9. 0 Feedstock, Not Screened 41. 3 49. 8 3. 4

1 Weight percent bone phosphate of lime (BPL).

2 Weight percent insoluble material.

3 Weight percent F9203 and A1 03.

4 To determine the BPL distribution,

(a) Determine the weight, T, in grams (b) Screen a 100 g. sample of of total BPL in a 100 g. sample of feedstock, said feedstock, and determine the weight, G, G, G", etc,

in grams, of BPL in each fraction, and (0) Calculate the distribution by the formulas:

100)( G lOOX G IOOX G" T T T m TABLE II.PRODUCT Weight Per- Assay Weight cent of Percent Material Total Feed Recovery Retained BPL 1 Insol- A & I 3 of BPL in Fraction able 2 Inch Fraction. 2. 0 72.1 7. 2.3 100 InchX-i-M Mesh Fra on-" 9. 0 74. 2 6. 4 2. 1 100 14 +325 Mesh Fraction 34. 7 76. 0 3. 5 l. 5 89. 7 Total Product 45.7 75.5 4. 2 1. 7 92 1 Weight percent bone phosphate of lime (BPL). 2 Weight percent insoluble material. 3 Weight percent F5203 and A120 W l i the beneficiation process of steps (a) through (e) by the 1. A substantially dry process for the beneficiation of phosphate rock which comprises:

(a) thermally drying a phosphate matrix while simultaneously subjecting said matrix to an attrition efiect within a vertical attrition column,

(b) pneumatically classifying the dried matrix to produce pebble phosphate and fine particles of phosphate rock, silica, and agglomerated clay,

(c) separating the pebble phosphate from said fine particles and recovering same as a final product,

(d) electrostatically separating said fine particles of phosphate rock from said fine particles of silica and agglomerated clay, and

(e) subsequently recovering the fine phosphate rock 7 particles.

2. The process of claim 1 in which the attrition effect is caused by conveying the matrix through a vertical column by flowing hot gases at a velocity of approximately 7,000 feet/minute and 350 F. through said column.

3. The process of claim 2 in which the pneumatic classification within said lwertical column separates said matrix into fractions of and inch, the inch fraction passing up the column and the inch fraction falling down said column to a fluidized bed chamber, thereby being in part disintegrated into inch material during said fall, said inch material subsequently being forced up the column and through gaseous fluid passing therethrough.

4. The process of claim 3 in which the matrix not disintegrated during said fall is removed from said fluidized bed, screened and recirculated to the attrition column.

5. The process of claim 3 in which the air-classification further separates the -200 mesh material from the inch matrix prior to separation of the pebble phosphate from said fine particles, and the 20 0 mesh material is subsequently collected and recovered.

6. The process of claim 1 in which the electrostatic separation produces a middling product which is recirculated and reprocessed.

7. The process of claim 5 in which the electrostatic separation produces a middling product which is recirculated and reprocessed.

References Cited UNITED STATES PATENTS 1,911,583 2/1935 Stockton 24118 X 2,197,865 4/1940 Johnson 20912 2,744,625 5/1956 Houston 209-12 3,241,774 3/1966 Jackering 24118 X WILLIAM W. DYER, 111., Primary Examiner.

ROBERT C. RIORDON, HARRY F. PEPPER, JR.,

Examiners. D. G. KELLY, Assistant Examiner. 

1. A SUBSTANTIALLY DRY PROCESS FOR THE BENEFICIATION OF PHOSPHATE ROCK WHICH COMPRISES: (A) THERMALLY DRYING A PHOSPHATE MATRIX WHILE SIMULTANEOUSLY SUBJECTING SAID MATRIX TO AN ATTRITION EFFECT WITHIN A VERTICAL ATTRITION COLUMN, (B) PNEUMATICALLY CLASSIFYING THE DRIED MATRIX TO PRODUCE PEBBLE PHOSPHATE AND FINE PARTICLES OF PHOSPHATE ROCK, SILICA, AND AGGLOMERATED CLAY, (C) SEPARATING THE PEBBLE PHOSPHATE FROM SAID FINE PARTICLES AND RECOVERING SAME AS A FINAL PRODUCT, (D) ELECTROSTATICALLY SEPARATING SAID FINE PARTICLES OF PHOSPHATE ROCK FROM SAID FINE PARTICLES OF SILICA AND AGGLOMERATED CLAY, AND (E) SUBSEQUENTLY RECOVERING THE FINE PHOSPHATE ROCK PARTICLES. 